The invention is directed to the field of vibroacoustics and, more particularly, to a structure for use in high-energy acoustic environments having low acoustically-induced vibration characteristics.
Spacecrafts such as satellites are placed in orbit using a dedicated launch vehicle. Spacecraft vibroacoustic environments and associated loads result from the launch vehicle rocket engines supersonic flow and the shearing of adjacent air. The acoustic energy significant to structural components is normally mostly within the frequency range of about 30-500 Hz. The apparent sound and reflections of this sound generally reach a peak level just at lift-off and then decay gradually as the launch vehicle increases speed and gains altitude. The initial engine noise is reverberated throughout the launch facility and is ultimately transmitted through the launch vehicle fairing where it impinges onto the spacecraft.
To reduce the level of acoustic energy to which the spacecraft is subjected, launch vehicles include a shroud which surrounds the spacecraft, protects against obvious wind loads, and absorbs some acoustic energy. Shrouds do not, however, provide sufficient acoustic protection of the payload. Measures are used to supplement the shroud and increase the acoustic protection of the spacecraft, including adding sound absorptive materials to the shroud. Although these materials add some acoustic protection, they also add undesirable weight to the launch vehicle and reduce lift capability.
The payload is mounted to the spacecraft bus. Honeycomb panels including imperforate graphite or aluminum facesheets on a honeycomb sheet core are commonly placed on or comprise the bus. Acoustic energy that impinges on the panels is reflected, absorbed or transmitted through the panels. The absorbed energy results in panel vibrational excitation. The transmission loss through a panel is related to the sound energy loss between an inlet face and an outlet face due to reflection or absorption. Known honeycomb panels exhibit this transmission loss and can be designed to maximize sound absorption so as to reduce noise.
Vibroacoustics are important for honeycomb panels that have appreciable surface area and are lightly loaded. Such panels are susceptible to acoustic excitation during liftoff, due to their high stiffness, low weight, low damping and high acoustic coupling factors. In general, panels with composite facesheets respond at higher levels than panels with facesheets of aluminum. The response of a panel is highly affected by its distributed mass loading. Lightly loaded panels have a much higher response than panels with loads equal to multiples of mass per square foot. Composite panels also exhibit less damping than aluminum panels due to bonded joints versus bolted joints, for example, which relates to their higher response. Because of weight constraints, spacecraft now utilize increasing numbers of composite parts in place of heavier metallic parts, oftentimes reducing the bus structural weight by as much as 20-30%.
For electronic assemblies that attach to equipment compartment panels, it is important to reduce vibrational exposure as much as possible. This is because electronic failures are most often linked to mechanical vibrations that produce failure conditions due to overstress or fatigue of the electronic components. Spacecraft equipment is specified to withstand vibrations in the 20-2000 Hz bandwidth, but is most susceptible to vibrations in the low frequency range of about 200-300 Hz. Rarely are the vibroacoustic levels high enough to actually damage the structural hardware such as the panels, but rather the electronic equipment are the weak link in the overall design. Acoustics drive the panels and cause vibration and significant additional acceleration of the electronics. The risk of electronics failure is greatest for a several second period during liftoff.
Known vibroacoustic reduction methods involve adding mass to the panels in the form of more structure, or using energy absorbing or dissipating devices. Adding mass or absorbing/dissipating devices achieves vibroacoustic reduction, but it also significantly increases the cost of launching the spacecraft associated with the added mass, and therefore is less than totally satisfactory.
Thus, there is a need for a panel structure for use in high-energy acoustic environments such as in spacecrafts that (I) allows reduced acoustic energy absorption and reduced acoustically induced structural vibration excitation of the structure and components to which the structure is attached; and (ii) is lightweight.